1. Technical Field
This invention relates in general to computers and, more particularly, to transmission of video signals in a computer.
2. Description of the Related Art
In the design of electronic devices, the transmission of electrical signals presents significant problems. A major concern is the electromagnetic interference (EMI) generated by each line carrying TTL (transistor to transistor logic) logic levels. The FCC (Federal Communications Commission) has provided strict guidelines on the allowable EMI emissions from an electronic device. In order to comply with FCC regulations, there have been many improvements made to the component housing designs which reduce EMI emissions.
However, as the internal circuitry of electronics become more complex, the number of lines increases along with the speed of data transmission on those lines. Both factors increase the EMI emitted by the internal circuitry. At some point, improvements to the housing design alone are not enough to comply with the FCC regulations.
Accordingly, manufacturers have used other techniques to reduce EMI emissions. One technique used in the portable computer field involves connecting the output of the video/graphics controller to an LCD (liquid crystal display) using LVDS (low voltage differential signaling). Differential signaling represents logic levels as the polarity between a pair of lines. For example, to represent a logical "1", the first line of the pair has a voltage of +0.345 volts, while the second line of the pair has a voltage of -0.345. To represent a logical "0", the first line of the pair has a voltage of -0.345 volts, while the second line of the pair has a voltage of +0.345. Because the logical levels are represented at low voltages, and at offsetting voltages, EMI is greatly reduced. This allows data from multiple lines to be multiplexed into fewer, higher speed, lines while still maintaining low EMI emissions.
Using LVDS transmission in other parts of the system has proven problematic. Because off-the-shelf LVDS/TTL converters affect the skew between the pixel clock and the signals, conversion of TTL/CMOS signals to LVDS with a subsequent conversion from LVDS signals to TTL/CMOS can result in setup and hold problems. The cost of designing custom LVDS/TTL converters for specific applications is generally prohibitive.
Therefore, a need has arisen for translating between LVDS and TTL/CMOS signals using off-the-shelf converters.